U.S. patent number 6,002,990 [Application Number 08/951,934] was granted by the patent office on 1999-12-14 for dynamic wavelength calibration for spectrographic analyzer.
This patent grant is currently assigned to Datex-Ohmeda, Inc.. Invention is credited to D. Alan Hanna.
United States Patent |
6,002,990 |
Hanna |
December 14, 1999 |
Dynamic wavelength calibration for spectrographic analyzer
Abstract
A novel process is disclosed for continually or periodically
correcting gas spectrometer measurements for wavelength calibration
drift without interrupting the gas measurement process. The
spectrometer employs certain calibration vectors for use in
obtaining single component information based on a composite
measurement for a multi-component gas sample. A calibration vector
can be provided for each component of interest as well as an
average spectrum and other spectra as desired. These calibration
vectors are used to selectively compensate for spectral
interference between the multiple components so as to effectively
isolate and measure a selected spectral characteristic for a
particular component of interest. Temperature changes and attendant
variations in spectrometer dimensions or other temperature related
factors can degrade measurement performance. The novel process
involves deriving wavelength correction data for the spectrometer
as a function of temperature, monitoring the spectrometer for
temperature changes during a time period of interest, and adjusting
or shifting one or more of the calibration vectors to compensate
for temperature variations. By performing temperature-related
adjustments based on the calibration vectors, the computational
intensity of the process is greatly reduced, thereby minimizing
processor burden/requirements. It is anticipated that the
correction process can be continually executed as a low priority or
background function in a multitasked system without requiring
dedicated processor resources.
Inventors: |
Hanna; D. Alan (Boulder,
CO) |
Assignee: |
Datex-Ohmeda, Inc. (Louisville,
CO)
|
Family
ID: |
25492352 |
Appl.
No.: |
08/951,934 |
Filed: |
October 16, 1997 |
Current U.S.
Class: |
702/88;
702/22 |
Current CPC
Class: |
G01J
3/28 (20130101); G01J 2003/2866 (20130101) |
Current International
Class: |
G01J
3/28 (20060101); G01N 021/25 () |
Field of
Search: |
;702/22-24,27,28,32,85-88,99,106,98 ;356/300-306,309 |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
Dally et al., Instrumentation for Engineering Measurements, pp.
17-20, Dec. 1993..
|
Primary Examiner: Hoff; Marc S.
Assistant Examiner: Miller; Craig Steven
Attorney, Agent or Firm: Holme Roberts & Owen LLP
Claims
What is claimed:
1. A method for use in compensating for variation of a selected
parameter in a spectrometer, comprising the steps of:
obtaining a calibration vector for at least one component of
interest with respect to a multi-component sample;
deriving wavelength correction data as a function of said selected
parameter for said spectrometer;
monitoring said spectrometer relative to said selected parameter
during a time period of interest;
based on a monitored value of said selected parameter, adjusting
said calibration vector using said derived wavelength correction
data; and
employing said adjusted calibration vector to obtain composition
information regarding said multi-component sample so as to identify
movement of a measured spectral characteristic.
2. A method as set forth in claim 1, wherein said step of obtaining
comprises receiving and storing a previously derived vector.
3. A method as set forth in claim 1, wherein said step of obtaining
comprises deriving said calibration vector based on measurements
obtained under known conditions using said spectrometer.
4. A method as set forth in claim 3, wherein said measurements
correspond to at least two values of said selected parameter.
5. A method as set forth in claim 1, wherein said step of deriving
comprises determining a relationship between said selected
parameter and measured wavelength values.
6. A method as set forth in claim 1, wherein said step of deriving
comprises obtaining measurements for known samples using said
spectrometer.
7. A method as set forth in claim 1, wherein said selected
parameter is temperature and said step of deriving comprises
obtaining a first measurement at a first temperature using said
spectrometer and obtaining a second measurement at a second
temperature using said spectrometer.
8. A method as set forth in claim 1, wherein said step of
monitoring comprises employing a sensor to measure said parameter
during said time period of interest.
9. A method as set forth in claim 1, wherein said spectrometer
comprises a laser and said identified spectral characteristic
corresponds to a plasma glow line.
10. A method as set forth in claim 1, wherein said identified
spectral characteristic comprises an extremum associated with a
known component of said multi-component sample.
11. A method as set forth in claim 1, wherein said step of
adjusting comprises shifting said calibration vector based on said
monitored value.
12. A method as set forth in claim 1, wherein said step of
adjusting comprises shifting said calibration vector based on a
change in said monitored value.
13. A method as set forth in claim 1, wherein said step of
employing comprises obtaining a composite measured spectrum for
said multi-component sample and mathematically processing said
composite measured spectrum using said adjusted calibration
vector.
14. A method as set forth in claim 13, wherein a calibrated
spectrum is obtained by said step of mathematically processing, and
said step of employing further comprises integrating said
calibrated spectrum to obtain a value related to a particular
component of said multi-component sample.
15. A method for use in compensating for variation of a parameter
affecting an analysis of a multi-component sample, said analysis
employing an isolation process for obtaining sample composition
information relating to individual components of the sample based
on a composite measurement of a multi-component sample, wherein
said isolation process involves a calibration vector and said
composite measurement for said multi-component sample, said method
comprising the steps of:
obtaining a calibration vector based on a calibration procedure,
wherein said calibration vector reflects a calibration value of
said parameter relative to said calibration procedure;
determining an actual value of said parameter, said actual value
being based on a measurement obtained at a time of interest
relative to said analysis; and
adjusting said calibration vector to compensate for a difference
between said calibration value and said actual value of said
parameter
wherein said step of determining comprises employing a spectrometer
to monitor movement of a measured spectral characteristic.
16. A method as set forth in claim 15, wherein said parameter is
temperature and said step of adjusting comprises shifting said
vector to compensate for a difference between a temperature
corresponding to the calibration vector and a measured
temperature.
17. A method as set forth in claim 15, wherein said step of
obtaining comprises operating a spectrometer to perform
measurements of known samples.
18. A method as set forth in claim 15, wherein said step of
determining comprises employing a sensor to measure said parameter
during said time of interest.
19. A method as set forth in claim 15, whether said spectrometer
comprises a laser and said spectral characteristics correspond to a
plasma glow line of said laser.
20. A method as set forth in claim 15, further comprising the step
of deriving measurement correction data as a function of said
parameter.
21. A method as set forth in claim 15, further comprising the step
of employing said adjusted calibration vector to obtain composition
information regarding said multi-component sample.
22. A method for use in calibrating a spectrometer for determining
composition information regarding a sample comprising the steps
of:
obtaining a first spectrometer measurement relating to a laser at a
known value of a given parameter;
employing said spectrometer to identify a first location of a
spectral characteristic relative to said first spectrometer
measurement;
employing said spectrometer to identify a second location of said
spectral characteristic relative to a second spectrometer
measurement, wherein said spectrometer is used to monitor movement
of said spectral characteristic; and
using said first location and said second location in an analysis
to determine said composition information regarding said
sample.
23. A method as set forth in claim 22, wherein said step of
obtaining comprises measuring illumination corresponding to a
plasma glow line of said laser.
24. A method as set forth in claim 23, wherein said step of
obtaining comprises selecting said plasma glow line such that said
plasma glow line is substantially free from interference from said
sample.
25. A method as set forth in claim 22, wherein said parameter is
temperature.
26. A method as set forth in claim 22, wherein said second
spectrometer measurement is taken at a time of interest relative to
analysis of said sample.
Description
FIELD OF THE INVENTION
The present invention relates to instruments for performing
spectrographic analyses of multi-component samples including
spectrographic gas analyzers for medical and non-medical
applications. In particular, the present invention relates to a
process for dynamically calibrating such an instrument so as to
compensate for variations in a system parameter or parameters that
may affect measured values. The invention is particularly apt for
correcting for temperature-related drifting of spectrometer
wavelength calibration.
BACKGROUND OF THE INVENTION
Spectrographic analyzers are used in a variety of medical and
industrial applications to analyze the composition of a gaseous or
fluid sample. One such application is the use of a spectrographic
gas analyzer to monitor respiratory, anesthetic and/or other
therapeutic gases in a patient respiratory flow line during a
medical procedure. The operation of such analyzers typically
involves illuminating a sample gas, measuring the illumination
intensity, or other value related to the illumination transmitted
through the sample gas, at various wavelengths, and analyzing the
measured values based on known transmission/absorption/scattering
characteristics of the components of interest to determine the
composition of the sample gas, e.g., the concentration of
components of interest in the sample gas. The number of wavelength
measurements employed can vary depending on system requirements,
but is generally at least as great as the number of components to
be analyzed. The set of wavelength-related measurements for a given
sample gas may be considered a composite measured spectrum.
The process for deriving individual component information from the
composite measured spectrum involves isolating effects due to the
component or components under analysis. In this regard, it will be
appreciated that various interfering components of the sample gas
may have overlapping effects such that the measured value at a
given wavelength may reflect effects due to more than one
component. The process for deriving individual component
information from the composite measured spectrum is well known in
the prior art, and involves the derivation of "calibration vectors"
which can be used to calculate the concentration of individual
components of the sample mixture. In general, there is one
calibration vector for each component to be measured, and the
measurement produced using each calibration vector will respond
only to the presence of a single component of the sample mixture.
The process for deriving individual component information therefore
involves at least a composite measured spectrum and one or more
vectors.
In order to obtain reliable information, it is important for the
spectrometer to be carefully calibrated. Such calibration involves,
inter alia, establishing and maintaining a known relationship
between particular measured values and corresponding wavelengths
such that the composite measured spectrum can be properly processed
using the predetermined vectors. It is known that various factors
can affect this relationship depending on the particulars of the
equipment. For example, a change in temperature may result in a
slight change in instrument geometry or dimensions which, in turn,
may cause a wavelength shift in the composite measured spectrum
unrelated to the composition of the sample gas. Such a shift, if
not accounted for, introduces an element of error into the system
and could result in significant hazards, particularly in medical
applications.
Two general approaches to addressing this source of potential error
are measurement drift prevention and measurement drift correction.
Measurement drift prevention attempts to avoid drift by maintaining
temperature or other parameters at a constant value. However, this
approach requires careful monitoring and control which increases
instrument complexity and can be impractical. Measurement drift
correction attempts to identify a drift of the composite
measurement spectrum due to temperature or other parameters, and
then correct the composite measurement spectrum based on the
identified drift. This approach, however, generally involves
substantial processing and dedicated processor resources,
particularly in applications involving high measurement rates and
multiple interfering components where the composite measurement
spectrum may have a complex form. These approaches have thus
resulted in substantial instrument complexity, processing
complexity and/or processor requirements.
SUMMARY OF THE INVENTION
The present invention is directed to a process for dynamically
calibrating a spectrographic analyzer so as to allow for accurate
multi-component sample analysis without the need to wholly prevent
wavelength drift or to adjust the composite measured spectrum to
account for drift. That is, the process of the invention operates
on an uncorrected composite measured spectrum in a manner that
yields accurate information regarding the composition of the
multi-component sample. The process involves adjusting a vector or
vectors based on an identified parameter value or parameter
variation, and processing the composite measured spectrum using the
adjusted vector(s) in a manner that yields composition information
largely independent of the position of the measured spectrum and
vector(s) relative to a wavelength range or corresponding
measurement field. The invention can be broadly viewed as
encompassing two aspects: (1) determining a relationship between
wavelength drift and variation of a system parameter and monitoring
an instrument for wavelength drift, and (2) using this relationship
and an identified drift to adjust a vector or vectors and process
the composite measured spectrum.
According to the first of these aspects, a relationship between
measured wavelength drift and variation of a system parameter can
be determined empirically. It should be noted that this "wavelength
drift" does not actually represent a change in illumination
phenomena but, rather, reflects changes in measured values due to
characteristics of the measurement equipment substantially
independent of the illumination source and sample composition. The
wavelength drift relationship is determined empirically by
successively operating the instrument at various values of the
parameter to be characterized (e.g., at various temperatures) and
identifying a spectral characteristic (e.g., a peak intensity)
associated with a known wavelength value. Once this relationship is
established, the same spectral characteristic can be monitored
continually or periodically at a time of interest relative to a
sample measurement to identify a wavelength drift or,
alternatively, the parameter can be directly monitored.
The particulars of this process can vary depending on the nature of
the spectrographic analyzer employed. In the case of laser-based or
Raman spectrometers, a laser main line or plasma glow line can be
employed for wavelength calibration. Such spectrometers operate on
a scatter principle, i.e., they are based on an understanding that
information regarding the composition of a sample can be determined
based on a direct or indirect analysis of laser illumination
scattered by the sample. Raman spectrometers therefore define at
least one main lasing wavelength and, in the case of gas lasers, a
number of plasma glow lines having known wavelength
characteristics. Such laser-related spectral phenomena can be used
for wavelength calibration prior to and during a measurement
process as described above. The plasma glow lines provide a
particularly useful reference for calibration purposes because
particular lines can generally be identified that are free from
interference by sample fluid spectral lines. These glow lines can
be monitored without interrupting the measurement process and
without complex processing to distinguish glow line effects from
sample fluid effects. In other types of spectrometers, a known
spectral characteristic such as a peak associated with a fluid
component can be monitored for calibration purposes.
According to the second aspect of the present invention as
identified above, a method is provided for compensating for
variations in an instrument parameter by adjusting a calibration
vector or vectors. The process includes the steps of obtaining a
calibration vector for at least one component of interest relative
to a multi-component sample, deriving wavelength correction data as
a function of a selected parameter for the instrument, and
monitoring the instrument relative to the selected parameter. In
the case of a Raman spectrometer, these steps may involve the use
of a laser main line or plasma glow lines as described above. In
other embodiments, for example, linear variable filter (LVF)
spectrometers, these steps may involve identifying or monitoring
particular characteristics of spectra during calibration or
measurement processes. The calibration vector is adjusted using the
correction data based on a monitored value of the selected
parameter. This adjusted vector is then employed to obtain
information regarding the composition of the multi-component
sample.
In one implementation of the present invention, an LVF based gas
analyzer for monitoring respiratory and anesthetic gases is
dynamically calibrated to compensate for changes in spectrometer
temperature. The gas analyzer includes a number of sensors
associated with the LVF such that the sensors provide measurements
over a wavelength range, i.e., the sensors define a measured
spectrum. Calibration vectors for various anesthetic agents and
other components of interest (and an average spectrum or other
spectra as desired) and temperature related correction data are
empirically derived based on measurements (i.e., spectra) obtained
under known conditions. The spectrometer temperature is then
monitored continuously or periodically during a medical procedure.
The temperature can be monitored directly by a sensor in the
spectrometer or indirectly by monitoring shifts of known
characteristics of the measured spectrum (e.g., movement of a
carbon dioxide line). During the medical procedure, the analyzer is
operated in conventional fashion to obtain measured spectra for use
in monitoring the components of interest. These measured spectra
are processed using the calibration vectors as adjusted based on
the monitored temperature and derived correction data to obtain
corrected calibration vectors. These corrected calibration vectors
are then used in the normal fashion to obtain information regarding
the composition of the sample gas. The process of correcting the
measurement vectors is preferably executed as low priority or
background function in a multitasked system, thus reducing or
eliminating the need for dedicated processor resources. In this
manner, accurate composition information is obtained despite
temperature fluctuations without shifting the measured spectra,
thereby reducing the computational complexity of the process and
minimizing processor burden/requirements.
BRIEF DESCRIPTION OF THE DRAWINGS
For a more complete understanding of the present invention and
further advantages thereof, reference is now made to the following
Detailed Description, taken in conjunction with the drawings, in
which:
FIG. 1 is a schematic diagram depicting an LVF spectrographic gas
analyzer in connection with which the present invention can be
implemented;
FIG. 2 is a schematic diagram depicting a Raman spectrographic gas
analyzer in connection with which the present invention can be
implemented;
FIG. 3 is a flow diagram/chart showing various implementation
options and details for the process of the present invention;
FIG. 4 illustrates a number of measured spectra and calibration
vectors such as employed in the calibration process of the present
invention;
FIG. 5 shows a number of measured spectra obtained at various
temperatures and thus illustrates temperature-related drift;
and
FIGS. 6A-6C illustrate a process for deriving temperature related
correction data.
DETAILED DESCRIPTION OF THE INVENTION
The present invention is directed to a process and related
structure for dynamically calibrating a spectrographic instrument
to account for variations of a system parameter affecting
wavelength related measurements. Such instruments are useful in a
variety of medical and industrial applications including, for
example, monitoring administration of respiratory and anesthetic or
therapeutic gases to patients and industrial effluent analysis. One
system parameter that can affect instrument performance is
temperature. In the following description, the invention is set
forth in the context of dynamic temperature related calibration of
gas analyzers for monitoring a patient respiration line during a
medical procedure. The present invention has particular advantages
for this application due to the critical accuracy concerns and
unpredictable temperature fluctuations characteristic of the
medical environment. However, it will be appreciated that various
aspects of the present invention are applicable to other systems
and applications.
FIGS. 1 and 2 are schematic diagrams showing two types of gas
analyzers that are used to monitor the administration of gases to a
patient during medical procedures. In particular, FIG. 1 shows an
LVF spectrographic gas analyzer and FIG. 2 shows a Raman
spectrographic gas analyzer.
Such gas analyzers are commonly used to monitor a variety of
components in the patient respiratory stream including, for
example, oxygen, carbon dioxide (CO.sub.2), Isoflurane, Enflurane,
Haloflurane, Desflurane, and Sevoflurane. The implementation of the
present invention may vary in some respects depending on the type
of spectrographic instrument employed. The instruments of FIGS. 1
and 2 are provided as two examples and it will be appreciated that
the invention can be implemented in other systems with appropriate
modifications.
Referring to FIG. 1, the LVF spectrographic gas analyzer is
generally identified by the reference numeral 100. Generally, the
analyzer 100 includes an illumination source 110, optics 120 for
directing illumination from the source through a sample chamber 130
to a detector surface, a filter/detector unit 140 and a processor
150. Additional components of the analyzer 100 that are not
relevant to the present description, e.g., housings, any reference
gas chambers and associated flow lines and optical choppers, have
been omitted for purposes of clarity.
The source provides illumination of a spectral range sufficient to
derive composition information for the components of interest of
the sample gas. In the illustrated embodiment, source 110 emits
infrared illumination across a wavelength range of at least about
4-12 microns, which allows for analysis of CO.sub.2 as well as a
variety of anesthetic agents. The optics 120 employed can vary
depending on a variety of factors including the geometry of the
instrument and may include lenses, filters and planar or curved
mirrors. Preferably, the optics 120 include a lens or curved mirror
for forming the illumination into a converging beam or imaging the
source 110 onto the focal plane of the filter/detector unit 140 so
as to enhance the optical efficiency of the instrument. It will be
appreciated that some or all of the optical components may be
located on the opposite side of the sample chamber 130 instead of
on the same side as shown.
The sample chamber 130 can be located, for example, in a
respiratory circuit flow line or in a side stream. In either case,
a sample is drawn into the chamber 130 via input line 132 and exits
the chamber 130 via exhaust line 134. In this manner, the sample
stream allows for continuous or periodic monitoring of the gas
composition during a medical procedure as desired. The sample
chamber 130 preferably has substantially transparent windows at the
forward and rearward walls to allow for transmission of the
illumination and is dimensioned and shaped to provide desirable
flow and optical pathlength characteristics. The gas exiting the
sample chamber 130 may continue in the patient respiration line, or
may be scavenged and vented to the ambient environment depending on
instrument design.
The illustrated filter/detector unit 140 includes an LVF filter 142
and a linear array 144 of detector elements 146. The LVF 142 has
filtering characteristics that substantially linearly vary from
side-to-side of the LVF 142 such that different wavelengths or
wavelength ranges of the illumination are passed at different
portions of the filter. For example, the LVF can be formed by
alternating layers of high index and low index of refraction
materials, where the thickness of the layers are varied to impart
the desired filtering characteristics. It will thus be appreciated
that the various detector elements 146 receive illumination of
differing wavelengths or wavelength ranges. More particularly, the
element 146 at one end of the array 144 defines the shortest
wavelength of the spectral range under analysis and succeeding
elements 146 define progressively longer wavelengths or wavelength
ranges, with the element at the opposite end defining the longest
wavelength of the range under consideration. Any suitable elements
146 may be employed. The illustrated elements 146 are pyroelectric
detectors that provide an electrical output proportional to the
incident illumination based on heat generated due to the incident
illumination. These outputs are transmitted to the processor 150,
which may be a microprocessor based computer. The corresponding set
of values defines a measured spectrum for the gas under
consideration and, in the case of a multi-component sample gas,
defines a composite measured spectrum. The processor 150 executes
the dynamic calibration process described in detail below, among
other things.
The number and type of elements 146 in the array 144 affects the
spectral resolution of the instrument and the applicability of the
dynamic calibration process described below. In the latter regard,
the calibration process of the present invention involves treating
the measured spectrum as a time-domain signal and conducting a
Fourier transformation on the spectrum/signal. As is well-known,
the applicability of such a transformation with regard to a
time-domain signal depends on the relationship between the sampling
rate of the signal to the frequency content of the signal. In
particular, the Nyquist sampling criteria specifies that the
following condition must be met in order to avoid losing signal
information:
where f.sub.c is the sampling rate and B is the maximum frequency
component of the signal.
In the present case, the Nyquist sampling criteria will be
satisfied for a given number of detector elements 146 if either the
gas under consideration has a sufficiently smooth spectrum or the
function defined by the detector element outputs (which is based on
but different from the gas spectrum) is sufficiently smooth. The
illustrated filter/detector unit 140 has approximately 70-80
pyroelectric detector elements 146 and has been found to provide a
suitable output function for Fourier analysis with respect to the
components of interest for respiratory and anesthetic gas
monitoring.
FIG. 2 shows a Raman spectrographic gas analyzer 200. The analyzer
200 provides composition information based on illumination
scattered by a sample gas. Generally, the analyzer 200 includes at
least one laser 210 for directing a beam of coherent illumination
through sample chamber and detector 220, and a processor 230. The
laser 210 may comprise, for example, a gas laser having a primary
lasing line or lines (in the case of a tunable laser). In addition
to these primary lines, the laser 210 emits a number of plasma glow
lines defining constant wavelength outputs. Sample chamber and
detector 220 includes sample chamber 226 and diffraction grating
228. Sample gas enters the chamber 226 through input line 222 and
exits through exhaust line 224 as described above. Illumination
scattered by the sample gas in chamber 226 is detected via
diffraction grating 228. The resulting output can be analyzed in a
well-known manner by processor 230 to yield composition information
regarding the sample gas.
As noted above, the implementation of the dynamic calibration
process of the present invention may vary in certain respects
depending on the type of spectrographic instrument involved. FIG. 3
illustrates various options and details of the dynamic calibration
process, which is generally identified by the reference numeral
300. The illustrated calibration process (300) is executed by the
processors as shown in either of the analyzers illustrated above.
It will be appreciated that the particular sequence illustrated may
be varied, and certain steps may be eliminated in accordance with
the present invention. The process 300 is initiated by obtaining
(310) calibration vectors for various components of the
multi-component gas sample. The calibration vectors are used to
assist in resolving interfering effects of multiple components to
yield individual component information. The calibration vector for
a particular component can be any set of values or algorithms that
can be applied to a composite measured spectrum to yield
information regarding an individual component or components.
One example of the vector function is illustrated graphically in
FIG. 4. Referring to FIG. 4, the curve designated A represents a
measured spectrum for an idealized component A. Such a measured
spectrum may be defined, for example, by the output of the LVF
filter/detector unit. Curve B represents a similar spectrum for
idealized component B. It will be appreciated that the forms of the
illustrated spectra have been simplified for purposes of
illustration. Similar diffraction related curves may be produced
using a scatter-based system. As can be seen, the measured spectra
A and B include a region of peak area overlap, R.sub.1, and a
region, R.sub.2, that is specific to component B, i.e.,
substantially free from overlapping interference. Curve V.sub.B
illustrates a vector that may be employed to isolate or distinguish
the effects due to component B from a composite measured spectrum
of sample A+B. For example, a product or sum of A+B and V.sub.B may
be employed to yield an adjusted spectrum where the effects of
interfering region R.sub.1 are minimized relative to the component
B specific region R.sub.2, thereby allowing for ready determination
of information regarding component B, i.e., concentration. In this
regard, the resulting adjusted vector can be integrated over a
wavelength range including R.sub.2 to yield a value corresponding
to a known composition of B as determined empirically through other
calibration processes. Similarly, a product or sum of A+B and the
vector for component A, V.sub.A, may be employed to determine
information regarding component A.
Referring again to FIG. 3, the process for obtaining such vectors
in the illustrated implementation involves measuring (311) known
samples under controlled conditions. In order to derive appropriate
vectors for use in distinguishing the effects due to the various
components, it is useful to know what the spectral response is for
the individual components under known conditions. Accordingly, a
particular instrument can be calibrated by obtaining measured
spectra for known concentrations of the individual components at
known temperatures. These measured spectra are analyzed to derive
appropriate vectors for the components of interest which are then
stored (312).
In one implementation of the invention, the process 300 also
involves deriving (320) wavelength correction data as a function of
temperature. In the case of the LVF spectrometer, for example,
wavelength measurements may drift as temperature changes due to
slight thermal expansions or other dimensional/geometric
variations. Such drifting is graphically shown in FIG. 5 which
shows several spectra obtained at temperatures ranging from
10-50.degree. C. It will be noted that the various measured peaks
and troughs vary as a function of temperature. This relationship
can be theoretically derived or can be empirically modeled for a
particular analyzer by performing measurements of a given sample at
two or preferably more temperatures so as to obtain wavelength
drift or correction data as a function of temperature. In the case
of Raman spectrometers or other instruments employing a laser,
temperature related drift can be identified by monitoring movement
of plasma glow lines. This correction data is then stored (322) for
later use.
One process for deriving wavelength correction data can be
understood by reference to FIGS. 6A-6C. The process is illustrated
in the context of an LVF spectrometer including multiple detector
elements for detecting illumination in the 7-10 micrometer
wavelength range. It will be appreciated that illustrated
temperature related wavelength shift is somewhat greater than
typical for purposes of illustration.
In particular, FIG. 6A illustrates a temperature related shift in
the relationship between the detector element number of the
detector array and the corresponding detected wavelength or
wavelength range. Such a shift may occur, for example, due to
slight changes in instrument geometry or dimension accompanying
temperature changes. Thus, as shown, illumination having a center
wavelength of about 9 micrometers may be received by approximately
element 32-33 at a detector temperature of 30.degree. C. and the
same illumination may be received by about element 51 when the
detector temperature is 20.degree. C.
FIG. 6B shows a corresponding shift of a measured peak for a given
sample. As shown, the sample includes a measured absorbance peak at
about detector element 51 for a detector temperature of 20.degree.
C. The same measured absorbance peak occurs at about detector
element 32-33 for a detector temperature of 30.degree. C. The same
peak shift is shown graphically in FIG. 6C where the detector
numbers corresponding to a detector temperature of 20.degree. C.
are shown above the peak curve and the detector numbers
corresponding to a detector temperature of 30.degree. C. are shown
below the curve.
The derived correction data compensates for such temperature
related shifts. In this regard, many different processes could be
utilized to derive the correction data. For example, for
applications where an approximation with a certain degree of error
can be tolerated, the temperature correction may simply entail an
algorithm or look up table for effectively shifting a vector or
vectors such that, for example, the vector value corresponding to
either detector element 32 or 33 for a detector temperature of
30.degree. C. is processed the same as the value corresponding to
detector element 51 for a detector temperature of 20.degree. C.
Greater accuracy may be obtained by incorporating a process for
linear or non-linear inter-element interpolation such that, for
example, a 20.degree. C. value for element 51 may be processed to
correspond to a 30.degree. C. value for a processed detector output
designated as element 32.6.
For greater accuracy, the process for deriving correction data may
involve a curve fitting analysis. In this regard, for example, a
first curve with a first known mathematical function can be fitted
to the various detector element values for a given sample at a
first detector temperature such as 20.degree. C. A second curve
with a second known mathematical function can be fitted to the
various detector element values for the same given sample at a
second detector temperature such as 30.degree. C. Similar functions
can be derived for other temperatures empirically or theoretically
mathematically. Correction data for a shift from a first
temperature to a second temperature can then be determined
mathematically based on knowledge of the corresponding functions
for the first and second temperatures. It will be appreciated that
such a curve fitting process increases accuracy by reliably
projecting inter-element values based on measured values, e.g.,
projecting peak locations that may occur between detector
elements.
During a medical procedure, the spectrographic analyzer is operated
in conventional fashion to obtain (320) composite measured spectra
for the sample gas. This may involve, for example, generating (331)
LVF detector outputs or generating (332) Raman spectrometer
outputs. In either case, such outputs comprise a series of values
that define a curve or spectrum. The detectors may be read out, for
example, at a frequency of 20 times per second. It will thus be
appreciated that measurement data is acquired and processed at a
substantial rate.
The spectrometer temperature is also monitored (340) during the
medical procedure to obtain drift related information. This can be
done directly or indirectly. Direct monitoring involves providing a
sensor or thermometer in or near the spectrometer to measure
temperature. The sensor then generates (341) outputs, continuously
or periodically, for use in accounting for expected wavelength
drift. Alternatively, a spectral analysis may be conducted (342) on
measured values to identify drifts that are indicative of
temperature change. For example, the location of a plasma glow line
may shift during a medical procedure indicating variation in
temperature or another parameter. Alternatively, an identifiable
spectral characteristic (e.g., a carbon dioxide peak) can be
monitored for drift. Such information can be used to account for
drift with respect to the measured composite spectrum as described
below.
Based on the monitored temperature or a monitored change in
temperature, the calibration vectors are adjusted (350) to account
for wavelength drift. This involves receiving (351) temperature
related information based on the monitored analyzer temperature,
retrieving (352) the stored calibration vectors and correction data
from memory, and deriving (353) adjusted calibration vectors by
adjusting the stored vectors based on the correction data and
monitored temperature. The adjusted calibration vectors are then
stored as new calibration vectors to be used until a further
temperature change is indicated.
The adjusted calibration vectors are then used to calculate (360)
component information. In this regard, the processor receives (361)
composite measured spectra, e.g., at a rate of 20 per second,
receives (362) the adjusted calibration vectors, e.g., periodically
or as required due to monitored temperature changes, and processes
(363) the composite measured spectra using the adjusted calibration
vectors to obtain composition values regarding one or more
components of interest. In one implementation, such composition
values are obtained by integrating the spectrum multiplied by the
calibration vector for a selected component over the spectral range
defined by the analyzer's detector output to obtain a value that is
proportional to a known concentration of the selected component. It
will be appreciated that the integrated value thus obtained is
substantially independent of the location of extema (peaks and
troughs) within the integrated range.
Upon reflection, it will be appreciated that adjusting the
calibration vectors rather than the composite measured spectra to
account for temperature-related wavelength shifts greatly reduces
the computational intensity of the calibration process. As note
above, composite measured spectra in a typical analyzer may be
obtained at a rate of 20 times per second and include a substantial
quantity of information. Temperature related adjustments to the
calibration vectors can be made at a much lower frequency, e.g.,
once every 20-30 seconds or as necessary, without unacceptably
compromising accuracy. Indeed, the processing required to implement
the calibration process of the present invention can be executed as
a low priority or background flnction in a multitasked computing
system such that processor burden is reduced and substantially no
dedicated processor resources are required for the calibration
process.
While various implementations of the present invention have been
described in detail, it is apparent that further modifications and
adaptations of the invention will occur to those skilled in the
art. However, it is to be expressly understood that such
modifications and adaptations are within the spirit and scope of
the present invention.
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